After left-wing lawmakers protesting at alleged election fraud forced outgoing Mexican President Vicente Fox to abandon a key speech, the BBC's Duncan Kennedy asks just how far the protests could go.

The protest shocked and amazed many Mexicans

Will they or won't they? It had been the hot political gossip of the week in Mexico City, a place that has turned speculation and rumour almost into a science.

Everyone was talking about whether the left-wing MPs would try to disrupt President Fox's last state of the nation speech or whether they would just sit in irreverent silence.

In the end they could not resist the opportunity.

On national television they decided their moment had come - the chance to secure some sort of leftist victory, however short-lived, over right wingers.

As stunts go, it was pretty impressive and quite well organised.

It all seems on a knife edge. If the leftists do not see any concrete results from their protests soon, what then

As though commanded by an invisible sergeant major, the MPs stood in unison and advanced on the stage in a kind of pincer movement.

Armed with photos and banners they took the high ground that was the podium and fired off salvos of abuse against the still absent President Fox.

"A traitor" they called him. It was an extraordinary sight - a kind of tactical victory after weeks of strategic retreat.

"I beg you to return to your seats," came the forlorn cry from the speaker of the House. The chanting continued.

"Vote by vote," the deputies shouted, the now familiar mantra of their long-running campaign to get all the votes in the recent presidential election recounted.

Raw reality

The Mexican people I was standing next to looked on with a mixture of shock and amazement.

"This is our national parliament being taken over", one told me, "I can't believe it."

Vicente Fox had to hand in his speech and depart

"Our president is being herded out of Congress," said another.

It was one of those rare moments where order meets chaos and the outcome has yet to be scripted. But this was no fictional drama. It was raw political reality.

Would the thousands of riot police stationed outside to keep away protesters come barging in? Would fighting break out among the deputies or with the president's bodyguards?

It must have been excruciatingly uncomfortable for the president, watching it all from the wings.

For days his aides had said he was determined to deliver his address in person - his political swansong to the nation.

The police presence was intimidating but violence was averted

Cool heads from his party advised against it, but he was not going to be deprived of his moment - an occasion to eulogise about his record in office and a platform to promote his successor, Felipe Calderon, almost certainly the country's next leader.

But sufficient numbers of those in the chamber were minded enough to spoil the party and so the president turned round, got into his car and made his speech from the safety of his soon-to-be-vacated presidential residence.

Was all this the start of a revolution?

Even sound-minded Mexicans have flirted with such thoughts for weeks now, ever since the mass street protests started a month ago following the bitterly contested result of the presidential election.

It is only six years since full multi-party democracy took root here and though the institutions of state appear to be robust, people will tell you they are not quite sure if things will descend into repression. Just like the old days.

Primetime stunts

With much of the capital city's main boulevard and central square still hidden under the canvas of camping left-wing demonstrators, there remains a sense here that the politics of the street have a role to play - even at the epicentre of parliamentary democracy.

The man who is inspiring the campers and the rowdy deputies is the darling of the downtrodden, Andres Manuel Lopez Obrador.

How far will Mr Lopez Obrador go to advance his cause?

All his political career this former welfare officer to Mexican Indians has used the streets to further his political and personal interests.

Some are already saying, who can blame him or the Left? Not much has gone their way in recent weeks.

On that July night they appeared to lose the election, despite their immediate claims of fraud.

Then international observers confirmed it had all been fair.

And then last week the courts delivered the legal and constitutional blow: the election had not been riddled with fraud.

Maybe the Left needed a fillip to tell their supporters they are still here.

Stunts, especially on primetime, are one of few options remaining to them.

Others say it is not as simple as that. In the current climate, with talk of setting up parallel governments, stunts can backfire and security forces can become involved.

As President Fox tried to make his speech to Congress, those riot police stood guard outside. Lots of them.

They were lined up behind a steel fence making sure the protesters on the other side did not break through. It was an intimidating sight.

"What are you doing?" I asked one particularly fearsome policeman in helmet, knee pads and with baton at hand.

He just looked at me and smiled. I thought, what if a brick, or worse, came flying over and hit the likes of him? Would he or they still be smiling? In the end nothing happened.

Continuing rumours

But it all seems on a knife edge. If the leftists do not see any concrete results from their protests soon, what then?

Can the moderates, including Mr Lopez Obrador himself, prevail?

Despite conciliatory sounds from Mr Calderon for the leftists to call it all off, Mr Lopez Obrador and his people will not let go. They are convinced of their cause.

But you come back to the question, will they or won't they take things to a more dangerous level? The scientists who feed the speculation and rumours of this place are hard at work.